(1) Balanced tertiary trisomics (BTT) are plants with an extra translocated chromosome that carry a dominant genetic marker gene near the translocation breakpoint. (2) The selfed progeny of BTT consist of 30% more BTT plants and 70% male sterile diploid plants, with the BTT used to maintain the stock and the diploids as the female parent in hybrid seed production. (3) Hybrid seed is produced by crossing the male sterile diploid female parent with a normal diploid male parent, with the seeds harvested being the desired hybrids.

1. BTT, Balanced lethals,XYZ systems

2. Balanced Tertiary Trisomics (BTT):
The term balanced tertiary trisomic has three words of
which
(1) “trisomic” indicates the presence of extra
chromosome,
(2) “tertiary” indicates that the extra chromosome is a
trans-located chromosome, and
(3) “balanced” refers to the breeding behaviour of the
trisomic.

3. • Ramage (1965) suggested the use of tertiary trisomics to supply
a male sterile source to be used as female parent for hybrid
seed production.
• “ BTT as a tertiary trisomic constructed in such a way that the
dominant allele of a marker gene, closely linked with the
translocation breakpoint of the extra chromosome is carried on
the extra chromosome, and the recessive allele is carried on the
two normal chromosomes that constitute the diploid
complement.
• The dominant marker gene may be located on the centromere
segment or the trans-located segment of the extra chromosome
Balanced tertiary trisomics for hybrid seed production

4. The dominant genetic marker (R) close to the breakpoint may be located
either on the centromere segment (a) or
the interchanged segment (b) of the extra chromosome in the BTT

5. Breeding behaviour of BTT:
BTT produces the following 3 types of functional gametes:
(i) ‘n’ Type which consists of both the normal chromosomes
and is functional on both male and female sides
(ii) ‘n + 1’ type consisting of 2 normal and one trans-located
chromosomes, functional on female side only
(iii) ‘n + 1’ type consisting of normal chromosomes

7. Selfing a BTT
• Selfed progeny of a BTT will produce 3 types of plants :
diploid, primary trisomic and BTT.
• The BTT will be identified by the presence of the
dominant marker.
• Transmission of the extra chromosome is nil through
pollen, while through the egg, it is transmitted, but with a
low rate.
• The progeny of BTT consists of 30% BTT and 70% diploid,
and a very low frequency of primary trisomics.
• Diploid and the primary trisomics in the progeny possess
recessive alleles of the marker gene.

8. •Breeding behaviour of a
selfed BTT, carrying a dominant
marker gene (R) in the trans
located segment of the extra
chromosome.
•It also carries the male
fertility gene (Ms) in the extra
chromosome , while the male
sterility gene (ms) in the
normal chromosomes.
•The progeny consists of male
sterile diploid plants, BTT, and
primary trisomics.
•The diploid (male sterile)
plants are used as female
parent in hybrid seed
production.

9. Use of BTT:
• Balanced tertiary trisomic is utilized for the production of
male sterile female parent to be used in hybrid seed
production.
• The BTT is constructed in such a way that the gene for male
sterility (ms) is located on the normal chromosome, while its
dominant allele (Ms) is located on the extra trans-located
chromosome.
• In this case, the selfed progeny will consist of BTT which
carries the Ms gene and are male fertile, while the diploids
will carry gene for male sterility (ms)

10. Production of Hybrid Seed:
• Crossing block consists of alternate strips of female and
male rows.
• The male parent (pollinator) is normal diploid, a
commercial cultivar, whereas the female parent is the
diploid progeny of BTT.
• Trisomics are weaker and late flowering and they may be
rouged to produce pure stand of male sterile diploids.
• In barley, seeds at a rate of 25-30 kg hectare are sown.
• At this seed rate, the trisomic plants are almost
completely eliminated by competition.
• The seeds produced in female rows are hybrid seeds

11. Method of commercial hybrid seed utilizing BTT and nuclear gene for male sterility (ms)

12. Maintenance and Production of BTT
• Seeds of BTT are produced in separate field, in the absence of
competition from male sterile diploids.
• For commercial production, in barley, the selfed seeds from
BTT are sown at a rate of 5-7 kg per hectare.
• The population consists of 30% BTT.
• Diploids are rouged at seedling stage on the basis of their leaf
characteristics (leaves of diploids are normal, while the
trisomics have long and narrow leaves).
• A large number of BTTs have been produced in barley
possessing the male sterile genes such as msg 1, msg 4,msg 6
and rn.sg 24 etc.
• In this crop, the first hybrid variety named “Hembar” was
produced in U.S.A. by Ramage and Wiebe in 1969.

13. Drawbacks of Balanced Tertiary Trisomics
• The production of Balanced Teritiary Trisomics plants is
complicated and laborious
• Balanced Teritiary Trisomics plants are generally weak, with
reduced pollen and female fertility which has bearing on
hybrid seed production.
• The extra chromosome is liable to be transmitted through
pollen,resulting in the appearance of diploid fertile plants in
the male sterile population.
• With the development of an effective cytoplasmic genic
male sterility system in barley, the Balanced Tertiary
Trisomics system has lost its significance.